Brake drums are conventionally manufactured by means of a number of differing processes and techniques. In one process commercially employed by applicants' assignee for manufacture of truck brake drums, a jacket is formed as an annular band of sheet steel. With the band externally fixtured, molten grey iron is centrifugally cast into the band to form a metallurgically bonded composite ring. After the annular drum ring has cooled, and with the ring externally fixtured, a cylindrical surface known as a rough bore is machined on the inwardly facing surface of the ring. Following this, a steel drum back or disc, having mounting openings preformed therein, is affixed to an edge of the drum ring, with the drum back mounting openings and the center of the rough bore of the ring located on a common axis. The radially inwardly directed surface of the brake ring is then finish-machined to form a braking surface as a cylinder coaxial with the drum back mounting openings. Numerous other processes for brake drum manufacture are known in the art.
In use on a vehicle or other applications, a brake drum should be balanced about its axis of rotation. Improper balance not only affects vehicle ride quality because of uneven distribution of centrifugal forces around the axis of rotation, but also greatly affects wear and operating life of the brake components themselves. Conventionally, individual finished brake drums are located on a balancing machine, and one or more weights may be affixed to the drum periphery to achieve weight-balance of the drum about is axis of rotation. However, such balancing techniques do not address an underlying problem of thickness variations of the brake ring around the circumference of the drum, which will lead to non-uniform brake wear, noise and vibration even if the drum is properly weight-balanced. Recognition of this problem has led some users, such as truck manufacturers, to specify to the drum manufacturer a maximum limit to weight that can be added to balance a drum and a maximum allowable thickness variation, resulting in scrap costs associated with drums that cannot be balanced within the maximum weight specification or meet the specified thickness variation limit.
It is therefore a general object of the present invention to provide a method and apparatus for manufacture of bodies of revolution such as brake drums that address the aforementioned problems in the art, and specifically that provide an as-manufactured brake drum having greatly enhanced weight-balance characteristics and thickness uniformity as compared with drums manufactured in accordance with conventional techniques. Yet another object of the present invention, is to provide a method and apparatus of the described character that satisfy the foregoing objectives and yet are economical to implement in manufacture of brake drums on a mass-production basis. A yet more specific object of the invention is to provide a method and apparatus for manufacture of brake drums that are substantially weight balanced about the axis of rotation of the drum back mounting openings following completion of the manufacturing process, and in which the brake ring is of substantially uniform thickness circumferentially of the mounting axis.